Determine the nature of the approximately 13.71 d−1 quiescent signal

Determine whether the coherent approximately 13.71 d−1 signal observed during the quiescent intervals of TESS Sectors 13 and 93 in the SU UMa-type dwarf nova Z Cha is a genuine negative superhump or an orbital sideband of the positive superhump, using longer, continuous, high-cadence observations to discriminate between the two interpretations.

Background

During the quiescent intervals of TESS Sectors 13 and 93, Z Cha exhibits a coherent signal near 13.71 d−1, corresponding to periods of approximately 0.0729 days and a negative superhump excess of about −0.02. Its frequency and period are consistent with a negative-superhump interpretation, and the signal persists after pre-whitening the orbital and positive-superhump components.

However, the simultaneous presence of strong positive superhumps permits an alternative explanation: orbital-frequency modulation of the positive-superhump pattern can generate an orbital sideband near the observed frequency. The available frequency evolution and period-excess statistics do not conclusively distinguish the two possibilities. Resolving the issue is important because confirmation of a negative superhump would bear on whether the coexistence of positive and negative superhumps in Z Cha can be explained by an eccentric disk with inner retrograde and outer prograde precession rather than by the classical tilted-disk model.

References

We therefore regard the NSH interpretation as partially supported observationally, but the current data cannot definitively exclude an orbital sideband origin for the ∼13.71 d−1 signal. Robust discrimination between the two scenarios requires longer, continuous, high-cadence time series covering additional quiescent epochs, which will enable precise tracking of signal coherence and frequency evolution and thus tighten the frequency-shift diagnostic.

— Eclipse Properties and Superhump Evolution in the SU UMa-Type Dwarf Nova Z Cha  (2609.10966 - Sun et al., 10 Sep 2026) in Section “The Nature of the ∼13.71 d−1 Signal: Negative Superhump or Orbital Sideband?”; see also Conclusions, third bullet